The invention is related to a digital telecommunication system for transmitting information between any one of a multiplicity of peripheral devices such as subscriber stations and data terminals under control of a central switch. In such systems, information transfer is performed by sampling the current condition of a peripheral device at a fixed sampling rate and by encoding the sample into digitalized information comprising a data word. Pulse code modulation (PCM) may be used for transmitting information in form of a serial data stream which is composed of frames each containing at least such a data word. Transmission lines are arranged between the peripheral devices and the central switch to accomplish the information transfer.
More specifically, the peripheral device connected to a transmission line enabling duplex communication with other peripheral devices may be subscriber stations, attendant consoles of a telephone switch or other recording and data transmitting units which comprise digital data terminals also using telephone lines for data transmission.
Conventional digital telecommunication systems are based upon two different transmission schemes for enabling duplex communication with sufficient transmission quality. In one scheme there is provided a plurality of transmission lines representing separate channels for useful information, such as speech or digital data, in each direction, for signalling and for synchronizing, wherein additionally for transmitting a direct current power supply at least one pair of wires of a transmission line is necessary. The second conventional transmission scheme is based upon a so-called "ping-pong" duplex data transmission procedure, that is data to be transmitted in opposite directions are alternatively generated in subsequent time periods of equal length and the two- or four-wire transmission line is alternatively used for transmitting information in the one or the other direction.
Since, in addition to binary coded information quite an amount of control information such as signalling information and synchronizing information has to be transmitted for control purposes between the central switch of the telecommunication system and the peripheral devices, it is desirable to organize the signal transfer such that control information and useful information can be transmitted simultaneously without any additional pairs of wires in a transmission line.
It is, therefore, an object of this invention to provide synchronizing means suitable for a digital telecommunication system transmitting useful information, such as digitalized voice samples, and control information simultaneously in one frame format. This means that a frame of the serial data stream is composed of a data word representing synchronization and signalling information and of at least one further data word containing the useful information.
Another object of the present invention is to provide a synchronizing circuit for detecting the alignment of pulse frames within a serial data stream under control of a system clock pulse train and for specifying time slots by means of respective enabling signals each determining, while activated, the occurrence of a specific time period wherein the respective data word of a frame is present.
Still another object of the present invention is to provide such a synchronizing circuit having a high quality performance which is achieved by a minimum outlay of electronic circuitry composed of commercially available circuits and components.